The Drosophila laboratory is currently engaged in five lines of research. (A) Many recombination-defective meiotic mutants cause, in addition to an overall reduction in crossing-over, a redistribution of exchanges such that genetic exchange is more proportional to physical length than in wild type. We are therefore examining the effects of recombination-defective meiotic mutants on exchange in a variety of chromosomal aberrations and also in the normally achiasmate chromosome 4, with the aim of elucidating the nature of the distributional changes caused by such meiotic mutants. (B) Two closely linked euchromatic autosomal genes, da and abo, were shown to cause maternal effects by interacting with specific (and different) X and Y chromosome heterochromatic regions. We have now found three new genes with these properties in this region. It thus appears that we have identified a special region containing a cluster of heterochromatin-regulating loci. We have in addition determined that in the case of abo there are two heterochromatic regions involved - one that operates in the abo female and the other that operates in her progeny. Finally, we have discovered a new X-linked, maternal-effect gene with the property that the progeny of mutant mothers die in proportion to how much heterochromatin (any heterochromatin) they carry. (C) The components of the Segregation-distortion system have been mapped and analyzed by deficiencies. (D) A new method for fate mapping lethal genes (focusing) has been developed and applied to an adult lethal. (E) Experiments on aging in Drosophila are being carried out.